1. Field of the Invention
The present invention relates to thermistors and methods for manufacturing thermistors, in particular, a thermistor based on a stack of a metal substrate, a thin-film thermistor layer, and electrodes, and a method for producing this type of thermistor.
2. Description of the Related Art
The known NTC thermistors or PTC thermistors for use as temperature sensors or other elements in protection circuits include the one disclosed in Japanese Unexamined Patent Application Publication No. 61-245502. The thermistor described in Japanese Unexamined Patent Application Publication No. 61-245502 has a flat-plate metal substrate that also serves as an electrode, a temperature-sensitive resistor film formed on either main surface of the substrate, and an electrode film formed on the temperature-sensitive resistor film.
In the structure of this thermistor, however, one of the electrodes is the flat-plate metal substrate and the other is the uppermost electrode film. Mounting this thermistor on a substrate or the like requires the electric connection to the electrode film to be established by wire bonding. Thus, it is impossible to mount this thermistor into an extremely small space. For example, when a thermistor of this type is used as a temperature sensor for an IC component packaged on a printed wiring board, there is a very narrow gap of 150 μm to 200 μm between the printed wiring board and the IC package and it is preferred to mount the thermistor into this gap. However, it is very difficult to mount an electronic component into such a very small gap by wire bonding.
According to Japanese Unexamined Patent Application Publication No. 61-245502, furthermore, the temperature-sensitive resistor film is formed on the wiring board by sputtering. Although the resistor film formed by sputtering is in good contact with the flat-plate metal substrate, sputtering is not suitable for mass production because performing sputtering requires costly production equipment. Solid-phase formation is desired; however, the use of this method results in the rate of shrinkage of the flat-plate metal substrate being different from that of the temperature-sensitive resistor film because of the different thermal expansion coefficients of the two elements. Even if the flat-plate metal substrate and the temperature-sensitive resistor film can be bonded together, therefore, the bonding strength of the interface may be insufficient, depending on the external environment.